A gain circuit, or amplifier circuit, may e.g. be used in an input interface of an electronic circuit, such as e.g. a receiver circuit, analog-to-digital converter, etc. The purpose of the gain circuit may e.g. be to amplify or restore a signal level of a received signal before further processing of the signal in the analog and/or the digital domain.
To obtain a relatively high linearity, the gain circuit may comprise an amplifier with high-gain, such as an operational amplifier (OP) or operational transconductance amplifier (OTA), connected in a feedback configuration. FIG. 1 is an exemplary circuit diagram of such a prior-art gain circuit. The gain circuit in FIG. 1 has a differential input terminals 2a and 2b, and differential output terminals 4a and 4b. An OP 6 is connected in a feedback configuration. A capacitor 8a is connected between the input terminal 2a and the positive input terminal of the OP 6. A capacitor 8b is connected between the input terminal 2b and the negative input terminal of the OP 6. A capacitor 10a is connected between the negative output terminal and the positive input terminal of the OP 6. A capacitor 10b is connected between the positive output terminal and the negative input terminal of the OP 6.
Resistors 12a-b and 14a-b are included in the gain circuit in FIG. 1 to provide a DC path to the positive and negative input terminals of the OP 6 from actively driven nodes, such as e.g. the output terminals of the OP 6. Thereby, charge can be transferred to and from the nodes connected to the input terminals of the OP 6 and a proper operating point for the input terminals of the OP 6 can be provided.
One problem with the gain circuit illustrated in FIG. 1 is that it may not be sufficiently linear for some applications, such as when the gain circuit is used as a pre amplifier in a high-resolution ADC. In particular, this may be a problem if a high signal bandwidth is targeted. As a nonlimiting example, in some video applications, a 10-bit resolution and up to 270 MHz sampling rate, or even higher, may be required. In such applications, the gain circuit depicted in FIG. 1 may be insufficient.
Furthermore, if the RC-constants of the parallel connections of resistor 12a and capacitor 8a, resistor 12b and capacitor 8b, resistor 14a and capacitor 10a, and resistor 14b and capacitor 10b are equal, a constant frequency response of the gain circuit in FIG. 1 is obtained (neglecting nonidealities in the OP 6). However, due to mismatch between components, said RC constants will normally not be equal. Therefore, the frequency response of the gain circuit may not be sufficiently flat for some applications.
In view of the above, there is a need for an improved gain circuit.